Small Craft Advisory 1005, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 PM AKST Sat Dec 17 2016

.ANALYSIS AND UPPER LEVELS...
There is a negatively tilted trough over southern Alaska moving
slowly towards the northeast. Precipitation with this feature is
diminishing from south to north as the trough axis passes. There
is a strong upper low over the central Aleutians vertically
stacked with a 942 mb surface low. Very strong winds are
associated with this low. There is a 200+ kt jet to the south of
the Aleutians. In the left front exit of the jet there is a triple
point low on the cusp of forming over the eastern Aleutians. In
between the Alaska trough and the Aleutian low there is a
transient short wave ridge over the Alaska Peninsula and eastern
Bering Sea.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement. There are some
differences in the handling of a surface low in the Gulf Sunday
afternoon into Sunday night, with the Nam appearing to be the
outlier with a central low further east. The Nam is much further
east than the other models with a low in the Gulf on Monday.
Further west, the Canadian has differences strait out of the gate
with the low near the central Aleutians this evening and tonight,
having the low much further east. The GFS/ECMWF were preferred.
Forecast confidence is near normal.

&&

.AVIATION...
PANC...VFR conditions and light winds are expected through Sunday
morning. North winds are expected to increase Sunday afternoon as
a strong low moves into the Gulf.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

As the current system continues to move across Southcentral,
sheltered inland areas have seen fog develop this afternoon as the
warmer, more moist air mass moves over the much colder land
surfaces. This looks to be a common theme for the next couple of
days especially when any partial clearing can take place in these
inland locations. Occasionally, fog may become widespread and dense
especially in the most sheltered locations throughout the region
where winds are light.

The upper trough will slowly exit to the north tonight with
precip rapidly tapering off most areas by midnight. Low-level
flow will shift around to the north and strengthen out ahead of a
storm-force front moving in from the Aleutians and North Pacific
late tonight through Sunday. A strong jet stream combined with
cold air moving in behind this will lead to development and
deepening of a triple-point low over the northwest Gulf on
Sunday. Precipitation will rapidly move northward Sunday,
initially affecting the Gulf coast and Kodiak Island. The strong
offshore flow out ahead of it will keep the remainder of the area
dry and will also limit snow totals across the interior Western
Prince William Sound zone, including Turnagain Pass and Alyeska.
As the low weakens Sunday night there is a chance of light snow
across the Cook Inlet region as moisture advects over the
mountains and a separate shortwave trough moves up from the south.

Another post-frontal low develops over the western Gulf of Alaska
during the day on Monday. This will help to focus precipitation in
what will already be an unstable convective environment. For this
reason, isolated lightning strikes will be possible across much of
the Gulf on the south side of this low center with the stronger
showers. Strong storm-force winds with near-hurricane-force gusts
will also be possible just south of the surface low track in
conjunction with a strong jet streak.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Weak ridging will allow for patchy fog and stratus to continue
mainly to inland areas of the Southwest Mainland this evening.
This will then be quickly scoured out overnight as the front
moving through the Eastern Bering makes landfall. This front will
bring gusty east winds to much of Bristol Bay on Sunday,
especially to areas along the Aleutian and Alaskan Ranges which
will receive the strongest winds. Along with winds, a few inches
of accumulating snow will fall over Western Bristol Bay, in
particular from Dillingham north and west where some areas of
blowing snow are possible with gusty east winds. This front will
then weaken over the area Sunday evening, leaving residual snow
showers primarily to Bristol Bay into Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Most of the focus over the Bering Sea will center around the
developing hurricane force low approaching the Central Aleutians.
Cold air advection and pressure ridges around the western edge of
this low will bring west wind gusts up to 90 mph to the Adak and
Atka areas overnight tonight before diminishing during the day
Sunday. This low will weaken as it approaches the Eastern
Aleutians Sunday, bringing strong westerly winds from Dutch Harbor
into the AK pen through Monday morning. Behind this low, rain and
snow will spread throughout much of the Western Bering/Aleutians
into Monday.

&&

.LONG TERM FORECAST (Days 3 through 7...Mon night through Sat...

A complex area of low pressure over the extreme eastern Bering, Akpen
and western Gulf of AK Mon night will consolidate over the
northern Gulf Tue into Wed. While a ridge of high pressure builds
over the Bering, the Gulf low will drift and weaken before
dissipating Sat. The Bering high moves over mainland Ak and beyond
during this period, allowing a resumption of eastward moving
systems across the Bering. A second low will develop Tue night
east of Kamchatka, and move near Cold Bay by early Sat before
crossing into the northern Gulf. A third low will develop near
Kamchatka on Sat before making its way across the Bering.

The initial Gulf low will bring accumulating snow to coastal
mountains and the Aleutian Range, diminishing through Sat as the
low weakens. The second low will bring wind and mainly rain to
the eastern half of the Aleutian chain, and the Akpen through the
week. The final low will bring wind to the western Bering and
Aleutians with some rain and snow on Sat.

The run to run consistency demonstrated by the models remains an
issue. Confidence is low to moderate with the timing of the lows,
as well as the location of precipitation. The forecast utilized
the WPC ensemble solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 187. MARINE...Hurricane Force Warning...174
176 177  Storm Warning...119 120 130 131 132 138 139 170 172 173
175 178 Gale Warning...125 127 128 136 137 140 141 150 155 160
165 171 179 180 185.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...DS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1018 PM AKST SAT DEC 17 2016

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0900 PM     NON-TSTM WND GST ADAK STATION            51.84N 176.64W
12/17/2016  M90 MPH                             AK   C-MAN STATION

            HIGHEST WIND GUST OF THE STORM SO FAR

1236 PM     NON-TSTM WND GST NIKOLSKI                52.96N 168.82W
12/17/2016  M75 MPH                             AK   AWOS

            HIGHEST GUST OF THE STORM

&&

JPW